you can easily book rooms on ctrip or elong websites. you can pay upon check-in at most hotels.
you don't need to join a tour as it is pretty easy DIY.
funny you should ask, that's exactly what we did this summer. stayed the first night at best western huangshan, which is just 5mn. walk to the hotsprings. stayed at baiyun hotel at the top, near brightness top (summit), then back to best western the 3rd night. the best western was great. from what i've gathered, all hotels at the mountain top are similar in prices and quality, so you can choose whichever location works best for you. these do have a higher price for foreigners (which was RMB900 for our room), but it's clearly stated on the website, and seems to be standard for the main hotels with good reputation. when you consider the fact that everything has to be carried up the mountain by human labor, the price is quite acceptable. btw, everything else is the same price for locals and foreigners. we had no problems at all with our reservations. we did have to use a credit card just to guarantee our room at baiyun hotel, but all worked well. so basically we would recommend both hotels.
the first week of may is national holiday in china. the last couple of years they cut it down to just 3 days off starting may 1st, but not sure what will come next year. i would avoid the first week of may regardless.